1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a reactor coolant pump auxiliary seal for reactor coolant system vacuum degasification.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in MacCrum U.S. Pat. No. 3,522,948, Singleton U.S. Pat. No. 3,529,838, Villasor U.S. Pat. No. 3,632,117, Andrews et al. U.S. Pat. No. 3,720,222 and Boes U.S. Pat. No. 4,275,891 and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
Thus, the sealing assemblies in the reactor coolant pumps are designed to hold high positive coolant pressures. This fact has raised some concerns about possibility of damage being done to the reactor coolant pumps during reactor coolant system vacuum degasification. Procedures for vacuum degasification of the reactor coolant system are described in U.S. Pat. No. 4,647,425 to Battaglia et al, which is assigned to the same assignee as the present invention. Basically, in vacuum degasification of the reactor coolant system a vacuum or negative pressure is imposed on the system and thus on the reactor coolant pumps. This, in effect, pressurizes the pumps in reverse. One major concern is that reverse pressurization might draw the water used to cool the pump sealing assemblies back into the pump sealing assemblies by a reverse flow of the water through filters which might bring contamination in the form of dirt and foreign matter along with the water from the filters into the sealing assemblies. Then, when the pumps are restarted after conclusion of vacuum degasification, the sealing assemblies might become damaged by the presence of the contamination therein.
Consequently, a need exists for an effective way to prevent reverse pressurization of the reactor coolant pumps so as to eliminate these concerns about possible damage to the pump sealing assemblies.